The demand of miniaturization on communications apparatus and electronic apparatuses such as mobile phones and personal computers necessitates the miniaturization of antenna devices used therein. Thus, chip antennas comprising power-supplying electrodes and radiation electrodes on or in base substrates made of dielectric or magnetic materials have become used.
There are various systems for mobile phones, for instance, EGSM (extended global system for mobile communications) and DCS (digital cellular system) widely used mostly in Europe, PCS (personal communications services) used in the U.S., and various systems using TDMA (time division multiple access) such as PDC (personal digital cellular) used in Japan. According to recent rapid expansion of mobile phones, however, a frequency band allocated to each system cannot allow all users to use their mobile phones in major cities in advanced countries, resulting in difficulty in connection and thus causing such a problem that mobile phones are sometimes disconnected during communication. Thus, proposal was made to permit users to utilize a plurality of systems, thereby increasing substantially usable frequency, and further to expand serviceable territories and to effectively use communications infrastructure of each system.
Accordingly, multi-band systems utilizing two or more frequency bands with one antenna are increasingly demanded. For instance, according to the needs of making mobile phones multi-functional, demand is mounting on small multi-band antenna devices, such as small dual-band antenna devices for handling a cellular system (for instance, transmission frequency: 824 to 849 MHz, receiving frequency: 869 to 894 MHz, though it depends on countries), a system for oral communications, and a global positioning system GPS (center frequency: 1575 MHz) having a position-detecting function, or small triple-band antenna devices for handling an EGSM system (transmission frequency: 880 to 915 MHz, receiving frequency: 925 to 960 MHz), a DCS system (transmission frequency: 1710 to 1785 MHz, receiving frequency: 1805 to 1880 MHz) and a PCS system (transmission frequency: 1850 to 1910 MHz, receiving frequency: 1930 to 1990 MHz).
As shown in FIG. 23, conventionally produced is a dual-band antenna device having two chip antennas disposed in parallel each comprising two radiation electrodes corresponding to two resonance frequencies (see, for instance, JP 11-4117 A). In FIG. 23, the antenna device 90 comprises a substrate 91, two chip antennas 93a, 93b mounted onto a surface 92a of the substrate 91, and a power-supplying electrode 94 and a ground electrode 95 formed on the surface 92a of the substrate 91. The ground electrode 95 and the two chip antenna 93a, 93b are close to each other. The power-supplying electrode 94 has one end divided to two, each connected to each power-supplying electrode 96a, 96b of each chip antennas 93a, 93b, and the other end connected to a high-frequency signal source (not shown). The other end of each of the first and second radiation electrodes 97a, 97b formed on the substrates of the chip antennas 93a, 93b is an open end.
However, the antenna device of JP 11-4117 A is not suitable for sufficient miniaturization because it comprises two chip antennas in a shape of rectangular parallelepiped. Though it has been proposed to mount a chip antenna 93b on a rear surface 92b of the substrate 91 for miniaturization, it does not meet the demand of thinning, because the thickness of a mounting substrate hinders such demand. Further, the increase of an opposing area between the ground electrode 95 and the chip antenna 93a results in increase in electrostatic capacitance and thus decrease in bandwidth. Thus, the antenna device of JP 11-4117 A fails to satisfy the demands of miniaturization, space reduction and bandwidth increase.
U.S. Pat. No. 6,288,680 discloses a antenna device comprising a chip antenna comprising a radiation electrode formed on a substrate, a power-supplying electrode connected to one end of the radiation electrode, a terminal electrode connected to the other end of the radiation electrode, and a mounting substrate having this chip antenna mounted thereonto, on whose surface a radiation electrode is formed. Because of the connection of the radiation electrode of the chip antenna to the radiation electrode on the mounting substrate, this antenna device has a large effective length of a conductor and a strong radiation electric field, thereby achieving a high gain and a wide bandwidth.
The antenna device disclosed in JP 2001-274719 A comprises a chip antenna mounted onto a mounting substrate, and a notch-shaped slit in a ground portion between the chip antenna and an adjacent high-frequency circuit. The notch slit suppresses a high-frequency current from flowing from the chip antenna to the high-frequency circuit, improving radiation characteristics.
However, the conventional antenna devices are disadvantageous in failing to meet all of the requirements of miniaturization, space reduction and bandwidth increase. Though U.S. Pat. No. 6,288,680 proposes the bandwidth increase, it simply suppresses the deterioration of bandwidth in a low frequency band, failing to handle a multi-band system. The gain increase by the notch slit as in JP 2001-274719 A only limits a path of a high-frequency current flowing in the ground electrode, failing to provide the bandwidth increase and to make the system adaptable for multi-band.
When pluralities of radiation electrodes are formed in the conventional antenna substrate to make the system adaptable for multi-band, it is difficult to keep isolation because of electrostatic capacitance generated between the radiation electrodes. Specifically, the higher the electrostatic capacitance between the radiation electrodes, the more the high-frequency current flows in the radiation electrodes in opposite directions, so that the radiation electrodes weaken the radiation of an electromagnetic wave each other, resulting in decrease in the gain (sensitivity). Though a wide band and a high gain are desirable in pluralities of frequency bands in multi-band antenna devices, JP 11-4117 A and U.S. Pat. No. 6,288,680 fail to provide any discussion on such points.
Much attention is recently paid to the reduction of influence of electromagnetic waves radiated from mobile phones, etc. on human bodies (heads) for health, and therefore antenna devices having low specific absorption rates (SAR) of electromagnetic waves are desired.